By Giulia Habib Meriggi, Sabine Donders, Candela Iglesias Chiesa, and Julius Okanta.

Ever wondered why some people seem invincible against illnesses while others catch every bug in town? Or how certain communities resist specific diseases even when not every member has developed immunity? And how epidemiologists track the impact of different diseases in specific groups of people?

In this article, we discuss three fundamental concepts in public health epidemiology: the epidemiologic triad, herd immunity, and attack rates.

Unravelling disease transmission: the epidemiological triad

This framework examines three crucial factors shaping disease transmission:

1. The Agent. Often called the ‘bug’ or ‘pathogen,’ it’s the microorganism causing the illness. Understanding the agent’s characteristics is essential—how fast it adapts, mutates, and spreads. Monitoring the pathogen is key to staying ahead of outbreaks.

2. The Host. The person, animal, or insect infected by the agent. Factors like age, genetics, and lifestyle influence vulnerability and transmission. Recognizing these factors helps design effective policies.

3. The Environment. External factors, collectively known as the environment, like climate, water quality, living conditions, and social interactions, impact disease spread. These factors can be either a risk or a protective factor and are an opportunity for intervention.

Herd Immunity : also known as “the Shield”

Herd Immunity, also known as Collective or Population Immunity, refers to a situation where enough of a population is immune to a disease, reducing the risk of infection within the population, and thus protecting those that are not (yet) immune.

Immunity can result from previous exposure or vaccination, making vaccines crucial for public health. Herd immunity protects the entire population by minimizing the possibility of transmission.

When can herd immunity be declared in a population?

It depends on what we call the ‘threshold,’ a percentage determining how much of a population needs to be immune to prevent sustained disease transmission. The threshold varies by disease, as some pathogens spread more effectively than others. For example, measles has a higher threshold than diphtheria, as it is more contagious.

Once the threshold has been achieved, herd immunity has been reached in that particular population. To maintain herd immunity, the percentage of people immune through vaccination and/or exposure to the disease must be kept at that threshold level over time.

Why is herd immunity important?
Herd immunity is important because it provides protection to vulnerable people in the population, particularly those who are immunocompromised or those who cannot be vaccinated for whatever reason (e.g. very young babies). By making the disease less prevalent, it lowers the risk of infection for the whole population, which for some is the only shield they have against the disease.

Following the Attack Rates

The attack rate, sometimes called the incidence proportion, is the proportion of people exposed to a pathogen (or other risk) that become ill. Think about it as the equivalent of the number of soldiers harmed by a sword among all the soldiers on a battlefield (i.e., those that were ”exposed” to swords).

The attack rate measures the risk of developing a specific disease in a defined period of time within a population over the same period.

It is calculated as follows:

Why is the attack rate important?

Attack rates are important for several reasons in the context of epidemiology and global health:

Monitoring Disease Trends: Attack rates help global health professionals and epidemiologists keep an eye on disease trends, which allows them to detect outbreaks early, whether it’s a localised foodborne illness or a global pandemic like COVID-19.
Assessing Intervention Efficacy: Public and global health professionals can use attack rates to evaluate interventions like vaccination campaigns and see how effective they are at reducing disease transmission.
Resource Allocation: In resource-limited global health settings, attack rates help inform the distribution of limited resources, ensuring that interventions are targeted where they are needed most and work best.
Risk Assessment: Attack rate calculations allow epidemiologists to assess the risk of disease spread, which enables health policy professionals to develop appropriate preventative and proactive measures in the case of an eventual outbreak.
Research and Epidemiological Analysis: Epidemiologists can compare attack rates of outbreaks across different populations and locations to figure out if there are any factors that are influencing disease spread.

The epidemiological triad, herd immunity, and attack rates are just some of the many epidemiological concepts and indicators used by public and global health professionals. Share in the comments if there are any other health indicators you would like to know more about!

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